I. Field
The following description relates generally to wireless communications, and more particularly to selecting a desired demodulation scheme at a receiver system.
II. Background
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Respective terminals communicate with one or more base stations via transmissions on forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-signal-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antenna are utilized.
A MIMO system supports a time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the access point to extract transmit beamforming gain on the forward link when multiple antennas are available at the access point.
The most popular MIMO-OFDM receiver architecture is the linear Minimum Mean Square Error (MMSE) spatial equalizer. However, the recently-developed list sphere decoding (LSD) significantly reduce the MIMO decoding error probability, especially when the channel code rate (ratio of number of bits entering a signal encoder to number of bits exiting the signal encoder) is high, modulation order (which represents a number of (coded) bits transmitted using a modulation symbol) is low, and the number of candidates in the list is large, at the cost of increased receiver complexity. In order to reduce complexity, a simplified version of LSD called QRM-MLD with quandrant detection method was proposed. According to the complexity analysis (in terms of number of multiplications), the QRM-MLD with 20-30 candidates has the complexity comparable to that of the MMSE spatial equalizer for the 4 transmit and 4 receive antennas. Furthermore, the complexity of the QRM-MLD increases in proportion to the number of the candidates.
The LSD may be applied for various purposes. First, the LSD can enhance the throughput performance of the single codeword (SCW) based MIMO receiver (i.e., a high-end SCW MIMO application). Second, the LSD can minimize the throughput loss of the multiple codeword (MCW) based MIMO receiver when the receiver does not employ a successive interference cancellation (SIC) (e.g., a low-end MCW MIMO application). In fact as the memory requirement and operational complexity of the SIC receiver incorporated with HARQ operations are quite challenging, the LSD would be very useful if it could achieve the throughput performance comparable to the SIC receiver with a moderate number of candidates. A need in the art exists for selecting the optimal demodulation scheme in light of transmit format (e.g., code rate and modulation order) characteristics of received data.